A new electrified high speed rail line linking London, Birmingham and Heathrow will form the centrepiece of a major government transport initiative today that will include a third runway at Britain's largest airport.

Gordon Brown will defy environmental critics and the Conservative party when the government formally unveils the package that will herald what ministers are dubbing a "green Heathrow".

The most dramatic element will be the building of a railway hub at Heathrow which will form part of a new 200mph rail line, running parallel to the congested west coast mainline, linking London and Birmingham direct, with a spur to Heathrow from St Pancras station, linking the new line to Britain's current high speed Eurostar line to Paris and Brussels.

The government will declare that it is delivering a new high speed link for the whole of the west coast mainline to the north-west of England even though the new line will stop at Birmingham. This is because most of the congestion occurs around Birmingham, allowing new trains to continue at speed, though not the full 200mph, to Manchester and Liverpool even after they come off the new line.

Greenpeace and the Tories last night pledged to maintain their opposition to Heathrow's third runway, with the environmental group vowing to ensure it would not be built. But Brown believes the announcement will trump the Tories. David Cameron told his party conference last year a Tory government would abandon a third runway and instead build a rail link between London, Birmingham and Leeds.

Geoff Hoon, the transport secretary who will outline the package to MPs today, will spell out a series of measures to create a "green Heathrow". These will include:

• Limiting new landing and takeoff slots on Heathrow's third runway to airlines that meet the strictest emissions standards.

• Assurances that EU rules on air and noise pollution will not be breached by the expansion of Heathrow.

These were the key areas of concern raised by Hilary Benn, the environment secretary, who is now supporting the announcement. The new rail link to the airport, which is designed to reduce the number of passengers arriving by cars, taxis and buses, will be highlighted as a key measure in ensuring that air pollution standards are met.

Greenpeace said the environmental "sweeteners" did not go far enough. John Sauven, its executive director, said last night: "It will shred the last vestiges of Brown's environmental credibility. An expanded Heathrow would become the single biggest emitter of CO2 in Britain. Labour MPs will lose seats over this. We'll fight it every step of the way because the lives of millions of people depend on us all slashing carbon emissions."

The prime minister is determined to press ahead with the expansion of Heathrow because he sees it as the kind of vital infrastructure project Britain should be building in a recession and because he believes it provides a perfect "dividing line" issue with the Tories.

Lord Adonis, the railway minister, has said he is keen on a £4.5bn hub to improve London's links with the north of England and Scotland. Plans for a new rail hub at Heathrow have already been drawn up by Arup, the engineering consultancy. They have proposed building a 12-platform station, linking the airport directly to London, the north west and the south west.